In lithographic printing, ink-receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink-receptive regions accept the ink and repel the water. The ink is then transferred to the surface of a material upon which the image is to be reproduced. Typically, in a method known as “offset”, this is done indirectly by first transferring the ink to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A class of imageable elements called printing plate precursors, useful for preparing lithographic printing plates, comprises a layer applied over the surface of a hydrophilic substrate. The layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, or in addition, the binder itself may be radiation-sensitive. The layer is commonly applied as a coating, using a solvent. Many positively working, thermally sensitive plates also include a surface layer that exhibits resistance to developer action.
During exposure this surface layer is destroyed in the exposed areas. After exposure to radiation the exposed regions of the coating are removed in the developing process, revealing the underlying hydrophilic surface of the substrate. Such a plate precursor is referred to as “positive-working”. The regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and repel ink. Recent developments in the field of printing plate precursors deal with radiation-sensitive compositions that can be imagewise exposed by means of lasers or laser diodes. This type of exposure, known as digital imaging, does not require films as intermediate information carriers since lasers can be controlled by computers.
Thermally imageable elements useful as lithographic printing plate precursors, exposable by infrared lasers or laser diodes as described above, are becoming increasingly important in the printing industry. Generally speaking, after imagewise thermal exposure, the rate of removal of the exposed regions by a developer in positive-working elements is greater than the rate of removal of the unexposed regions, so that during development the exposed regions are removed by the developer to form an image.
Imaging of digital, thermally imageable precursors is typically done using platesetters, where the plate precursor is mounted either
i). on a rotatable drum (external drum), typically using clamps, or
ii). in a drum (internal device), in which case the plate precursors are held in place with compressed air or with clamps, which may be magnetic.
When a positive-working lithographic printing plate precursor is imaged on a platesetter employing clamping devices for holding the precursor onto the outside surface of an exposure unit, the clamping device prevents the successful exposure of the coating immediately under it. After development, this unexposed area of coating accepts ink. Unless this section of coating is removed manually (a time-consuming process), it will cause an unwanted image on the press. The problem is particularly troublesome for web presses, where ink is wasted and unwanted inked image areas can transfer to the back of paper stocks.
Rather than using clamps, some platesetters employ suction cups and powerful vacuums. On mounting a plate precursor on such a platesetter, however, at least one edge of the plate precursor is typically inserted into a crevice in the drum, where it is shaded from the imaging radiation. In such systems, the presence of unwanted, remaining image areas is therefore still not avoided. Thus there remains a need for ways of avoiding the time-consuming step of removing such unwanted image areas after plate development.